1. Field of the Invention
This invention relates to the use of diurethanes as stabilizing additives in sealing compounds.
2. Statement of Related Art
Sealing compounds are widely used in practice in the assembly of objects of various kinds. They can be hardened by moisture hardening in wich isocyanate groups enter into a crosslinking reaction as reactive terminal groups of prepolymers. It is also known that plastisols can be used for similar purposes, in which case hardening is carried out by a thermal reaction. However, systems of this type can also gel and ultimately harden on their own over prolonged periods either at room temperature or at moderately elevated temperature.
One of the disadvantages of these moisture-hardening systems is that, even in the substantial absence of air and moisture, they are not sufficiently stable. They will undergo an increase in viscosity through crosslinking after a few months to such an extent that they can no longer be processed by the usual means.
High-quality polyurethane systems, of the type used as sealing compounds in the building and automotive industry and in ship building, often contain swellable PVC powder as a thixotropic filler. Compared with other known fillers, such as chalk, silica and carbon black, this filler has an extremely low water content which does not adversely affect the storability of the isocyanate-containing compositions. In addition, it provides the end product with excellent rheological properties. The swelling agents and plasticizers used for PVC are, primarily, alkyl benzenesulfonic acid esters or phthalic acid esters, for example dioctyl phthalate. However, the above isocyanate systems have the disadvantage that the combination of swollen PVC and platicizer tends to continue swelling in the composition, particularly when it is stored at relatively high temperatures. As a result, there is a considerable increase in viscosity which interferes seriously with processing.
In attempts to prevent this continued swelling with standard stabilizing additives, such as polyisobutylene for example, it was found that, through the incompatibility of these apolar compounds with the isocyanate prepolymers, migration of these substances adversely affected the appearance of the hardened compositions.
Another disadvantage of these polyurethane systems is that, due to the viscosity of the prepolymers used, it is only possible to use a relatively narrow, preferably low molecular weight range, because prepolymers of relatively high viscosity cannot be diluted with the plasticizer normally used because a relatively high plasticizer content would adversely affect the swelling behavior of the PVC. However, it is precisely high-viscosity prepolymers which show the desired, particularly favorable mechanical properties after hardening. Accordingly, there is a need to solve the above problems.